A Brief History of Astronomical Imaging Systems

Transcription

1 A Brief History of Astronomical Imaging Systems 1

2 Oldest Imaging Instruments circa 1000 CE 1600 CE Used to measure angles and positions Included No Optics Astrolabe Octant, Sextant Tycho Brahe s Mural Quadrant (1576) Star Catalog accurate to 1' (1 arcminute = 1/60 limit of resolution of unaided human eye) Astronomical Observatories were built by church as part of European Cathedrals (possible subject for course term paper) 2

3 Early Imaging System the Mural Quadrant Most accurate positions of stars and planets then available Used by Johannes Kepler to derive the three laws of planetary motion Laws 1,2 published in 1609 Third Law in 1619 H.C. King, History of the Telescope 3

4 Kepler s Three Laws of Planetary Motion 1. The orbits of planets are ellipses with the Sun at one focus. 2. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse, thus the planet travels faster when it is closer to the Sun. 3. The ratio of the squares of the periods ( years ) for two planets is equal to the ratio of the cubes of their semimajor axes. 4

5 Optical Instruments, (1609+) Refracting Telescope uses lenses to redirect light Invented in 1608 Hans Lippershey (1570? 1619) Early Use in Astronomy 1609, by Galileo Galilei ( ) Johannes Hevelius ( ) Reflecting Telescope Invented ca Isaac Newton ( ) Spectroscope Invented ca. 1669, also by Newton 5

6 Galileo s s Telescopes Cracked Objective Lens Combination of Two Lenses Objective Two surfaces: flat and convex Eye Lens (Ocular) Two surfaces: flat and concave Magnified by 20 6

7 Galilean Telescope f objective Ray incident above the optical axis emerges above the axis image is upright Small Field of View 7

8 Galilean Telescope θ θ Ray entering at angle θ emerges at angle θ > θ Larger ray angle angular magnification 8

9 Keplerian Telescope f objective f eyelens Ray incident above the optical axis emerges below the axis image is inverted 9

10 Keplerian Telescope θ θ Ray entering at angle θ emerges at angle θ where θ > θ Larger ray angle angular magnification 10

11 Refractive Index Denoted by n Measure of ratio of velocity of light in matter to that in vacuum c n = v c = velocity in vacuum meters/second v = velocity in medium measured in same units n

12 Sample Refractive Indices Vacuum: n = 1.0 Air: n Water n 1.33 Glass: 1.71 n

13 Lenses Redirect ( Bend( Bend ) ) Light by Refraction due to Different n Incident Ray θ 1 -θ 1 Reflected Ray n 1 n 2 θ 2 Refracted Ray 13

14 Refracted Angles Determined by Snell s s Law n sin θ = n sin θ n n L 1 1 θ2 = sin N M 2 sin θ 1 O QP 14

15 Problem with Refracting Telescopes: Optical Dispersion Refractive index n of glass is not constant n of glass tends to DECREASE with increasing wavelength λ Refracted angles change with wavelength λ Focal length f of lens tends to INCREASE with increasing wavelength λ Different colors focus at different distances Chromatic Aberration 15

16 Optical Dispersion n Ultraviolet λ Infrared 16

17 Chromatic Aberration 17

18 Minimize Chromatic Aberration Chromatic aberration is less noticeable for lenses with long focal lengths f 18

19 Hevelius Refractor, ca Objective Lens Eye Lens H.C. King, History of the Telescope 19

20 Later Methods to Diminish Chromatic Aberration Create lens systems from multiple lenses made from different glasses doublets or triplets Designed so that chromatic aberrations cancel for some wavelengths Difficult to design and fabricate Beyond capability of early optical technicians 20

21 Achromatic Lenses Achromatic means no color Two (or more) lenses with different glasses Crown, with smaller n Flint, with larger n 21

22 Easy Way to Eliminate Chromatic Aberration Don t use lenses!! All colors focus at same distance f f 22

23 Newton s s Reflector ca "-diameter mirror no chromatic aberration mirrors reflect all wavelengths at the same angle! H.C. King, History of the Telescope 23

24 Large Historical Reflecting Telescope Lord Rosse s 1.8 m (6'-diameter) telescope metal mirror, 1845 H.C. King, History of the Telescope 24

25 History of Imaging Sensors Eye Limited sensitivity Limited range of wavelengths Images can be stored only by hand (drawings) Image Recording Systems Chemical-based Photography wet plates, dry plates, Kodak plates, Physics-based Photography, Electronic Sensors, CCDs 25

26 History of Imaging Sensors Groundbased Infrared Imaging 1856: using thermocouples and telescopes ( one-pixel sensors ) 1900+: IR measurements of planets 1960s: IR survey of sky (Mt. Wilson, lead sulfide PbS detector) Spacebased Infrared Imaging 1983: IRAS (Infrared Astronomical Satellite) cooled Silicon and Germanium detectors 1989: COBE (Cosmic Background Explorer) 26

27 History of Imaging Sensors Airborne Infrared Observatories Galileo I (Convair 990), /12/1973 (crashed) Frank Low, 12" diameter telescope on NASA Learjet, 1968 Kuiper Airborne Observatory (KAO) (36"- diameter telescope) 27

28 Galileo I (Convair( 990) Started in 1965 Several ports available for cameras Crashed 4/12/1973 midair collision on landing at NAS Moffet 28

29 NASA Learjet, " diameter telescope, by Frank Low 29

30 Kuiper Airborne Observatory Modified C-141 Starlifter 2/ /1995 ceiling of 41,000' is above 99% of water vapor, which absorbs most infrared radiation 30

31 Stratospheric Observatory for Infrared Astronomy SOFIA Boeing 747SP 2.7-m Mirror (106") 31

32 Spaceborne Observatories Orbiting Astronomical Observatory (OAO), 1960s Infrared Astronomical Satellite (IRAS), 1980s Hubble Space Telescope (HST), 1990 Chandra (Advanced X-ray Astrophysics Facility= AXAF), 7/

33 History of Imaging Systems for Radio Astronomy Wavelengths λ are much longer than visible light millimeters (and longer) vs. hundreds of nanometers History 1932: Karl Jansky (Bell Telephone Labs) investigated use of short waves for transatlantic telephone communication 1950s: Plans for 600-foot Dish in Sugar Grove, WV (for receiving Russian telemetry reflected from Moon) 1960s 305m Dish at Arecibo, Puerto Rico 1963: Penzias and Wilson (Bell Telephone Labs), Cosmic Microwave Background 1980: Very Large Array (= VLA) in New Mexico 33

34 Jansky Radio Telescope Image courtesy of NRAO/AUI

35 Large Radio Telescopes 100m at Green Bank, WV 305m at Arecibo, Puerto Rico Image courtesy of NRAO/AUI 35

36 Very Large Array = VLA 27 telescopes 25m diameter transportable on rails separations up to 36 km (22 miles) Image courtesy of NRAO/AUI 36